Wavelength-division multiplexing devices are of interest for the processing and distribution of signals in optical communication networks. One important technology for making such devices is silicon optical bench (SiOB) technology. In SiOB technology, planar optical waveguides, and devices based thereon, are fabricated on planar silicon substrates. A typical SiOB waveguide includes a lower silicon dioxide cladding, and a core and upper cladding of phosphosilicate glass deposited on the substrate and lithographically patterned. (SiOB methods are described, for example, in U.S. Pat. No. 4,902,086, issued to C. H. Henry, et al. on Feb. 20, 1990.) This technology is particularly useful for applications in the communication industry because it makes possible highly compact packaging of complex optical circuits, as well as integration of optical sources and detectors on the silicon substrates.
According to a second technology based on planar waveguides, the waveguiding components are manufactured form III-V semiconductor materials, rather than from silica-based materials. Aspects of this technology are described, e.g., in M. Zirngibl, et al., "Efficient 1.times.16 Optical Power Splitter Based on InP," Electron. Lett. 28 (1992) 1212-1213, and in M. Zirngibl, et al., "Demonstration of a 15.times.15 Arrayed Waveguide Multiplexer on InP," IEEE Photon. Technol. Lett. 4 (1992) 1250-1253.
Some designs for optical communication networks call for a wavelength-division multiplexer (WDM) capable of serving as a broadband, channel-dropping filter (or more generally, a channel adding/dropping filter). Such a device would, for example, have one input port and two output ports, to be denoted Ch. 1 and Ch. 2. In the absence of resonant effects, the input stream would be passed through to Ch. 2. However, input sub-channels having selected, resonant wavelengths would be directed to Ch. 1. By way of illustration, some proposed networks would carry communication signals in the 1.3 -.mu.m and 1.55 -.mu.m wavelength bands, and for fault-detection by optical time-domain reflectometry (OTDR), the same networks would carry diagnostic signals in the 1.42-.mu.m band. A WDM filter would be useful for separating the two signal bands from the diagnostic band.
However, in optical communication networks, it will generally be desirable to avoid the use of filters with sharply peaked passbands. That is, it is possible that the lasers used for transmitting communication signals will not be perfectly stable in wavelength. Instead, environmental factors such as temperature changes may cause the transmission wavelengths to vary by as much as a few percent from the design wavelengths. A practical communication network will need to be tolerant of these wavelength variations. Optical filters, such as WDM filters, will be tolerant only if they have passbands that are not too sharply peaked near the wavelengths of maximum transmission.
Until now, however, at least some WDMs that are implementable using planar waveguide technology have had undesirably sharp transmission peaks. One such device is a SiOB Mach-Zehnder WDM, based on adiabatic 3-dB couplers. This device is described in R. Adar et al., "Adiabatic 3-dB Couplers, Filters, and Multiplexers Made with Silica Waveguides on Silicon", J. Lightwave Technol. 10 (1992) 46-50. This device had a transmission peak, centered on 1.55 .mu.m, with a width at the 10-dB level of only 1%.
Another WDM based on planar waveguides is described in C. Bornholdt, et al., "Meander coupler, a novel wavelength division multiplexer/demultiplexer", Appl. Phys. Lett. 57 (1990) 2517-2519. The WDM described in Bornholdt is formed by patterning III-V materials deposited on an indium phosphide substrate. It employs periodic coupling between waveguides that differ in rib height, and therefore in effective refractive index.
One problem with this WDM is that it is relatively complex to manufacture. That is, after the waveguiding layer is grown, two etching steps are required to define the coupled waveguides. The first etching step defines the lateral dimensions of the waveguides, and the second step defines the difference in rib height between the waveguides. Moreover, the etching process used to define the rib heights has a typical accuracy no better than about 15%, and consequently it will be difficult for a manufacturer to maintain close tolerances on the center wavelengths of manufactured devices.
Thus, practitioners have until now failed to provide a WDM optical element that is relatively simple to manufacture to stringent tolerances, and that, in operation, will tolerate significant fluctuations of the transmission wavelengths.